Well the 7S looks like it more closely matches the min max of the board - which is 33V nominal down to around 21 nominal (its a bit wider actually to account for manufacturing variation). However what problem are you trying to solve with this? The 12350 battery that we recommend lasts for ~30+ hours of continuous use and we’ve not yet really been able to run it flat - why do you want to change to a custom pack made with 18650?

The people we’ve talked to that used custom battery packs on their robots found later on that they had terrible trouble getting battery pack replacements quickly when they needed them. You’ll need to replace the charger, put in a balancer as well as do lots of work. The 18650 is basically gasoline, lithium and electricity, I think its incredible that they don’t catch fire more often which is why they are not approved for many environments. Add in that you can’t really ship them legally by air and you are creating lots of headaches for yourself.

What are you trying to achieve with this change? We don’t recommend it.

The last customer that tried changes like this had a fire on their prototype robot a few days before a major demo with a huge customer and the company eventually went out of business - we’d like to make sure you don’t have a similar experience.

I wish to add to David’s comment in that we have not designed the product to work down to 18 volts. On the rev 5.0 controller boards we have set lowest voltage we will fully operate just below the threshold David has mentioned so we cannot say that it will work at all down at 18 volts which is below our operating design point. I have just worked out the numbers so that 18V is the cutoff point with worse case component tolerance so 6 18650 batteries is very much right at the limit and any dip would trigger our low voltage protection. Earlier boards were more generous and allowed lower battery voltage but we view that as a fault and not a guaranteed operating point. I do not recommend a 6-cell 18650 operating point and in fact it may not work at all on some units depending on part tolerance.